I don't remember how the energy system was before, but I like how it works now ! The balance between risk and reward is great, it adds a good pace to the game, I really love it !
I think what you can still improve is the general feedback on what the enemies are doing, for example you never know when the pink enemies will shoot, so it's very hard (and a bit too frustrating) to try and dodge their shots. With a bit more differentiated and predictable patterns you could reduce the amount of enemies and make the game a bit more readable. I think there is a lot of cool enemies mechanics that can synergize with the core mechanic. For example, make an enemy that has the same kind of weapon as the player, is invincible until he shoots, and goes to catch his weapon after a while, I think it can be really fun gameplay wise.
The boss is definitely too hard, it felt like it was impossible to hit him without taking damage, and it was a bit disappointing to not be able to hit him more than once in a row. I think he should shoot more in bursts to leave at least a small time window where you can try something without always being in a bullet hell. Just keep the same shooting patterns but make the boss stop for a few seconds once in a while, and it might become a bit more fun to play.